This proposal aims to study factors affecting the regulation of protein synthesis during fertilization, early cleavage and later embryogenesis. During development there is a sequential appearance of unique proteins synthesized on defined classes of polysomes. Also an early event following fertilization is the activation of the protein synthesizing system of the egg, binding of ribosomes to messenger RNA and formation of polysomes. Factors regulating the recognition of messenger RNA by ribosomes during fertilization and early embryogenesis will be studied. Emphasis will be placed on understanding the regulation of protein synthesis, especially the processes involved in specificity of binding ribosomes to messenger RNA. Specificity in binding brings about discrimination of messenger RNA to be translated and thus may regulate the sequential appearance of proteins during embryogenesis, post embryonic development and morphogenesis. BIBLIOGRAPHIC REFERENCES: Ilan, Joseph and Judith Ilan (1975). Similarities in properties and functional difference in purified leucyl-tRNA synthetase isolated from two developmental stages of Tenebrio molitor, Devel. Biol., 42, 64-74. Ilan, Joseph and Marcus Singer (1975). Sampling of the leucine pool from the growing peptide chain: Difference in leucine specific activity of peptidyl-transfer RNA from free and membrane-bound polysomes. J. Mol. Biol., 91,39-51.